One problem that the inventor has tried to overcome by means of the present invention is to reduce the supply of air to the water during the purifying process for removing protein from the water. If air is supplied to the water in the form of pressurized air or bubbles, much of the nitrogen gas in the air will be dissolved in the water. This may lead to undesired nitrogen content in the water. It is known that nitrogen dissolved in water inhibits fish growth, so it is desirable to keep the nitrogen content of the water sufficiently low so as to avoid inhibition of fish growth.
It belongs to the known art to remove particles and proteins from water by injecting air to form air bubbles in the water so as for the particles and proteins to bind to the bubbles and thereby be forced to the surface of which they may be skimmed off. The efficiency of this process depends on several factors, of which density and pH are important. pH is important as fresh water usually has a lower pH than salt water. This will reduce the electrical bindings formed between particles in fresh water. Traditionally, bubble formation for protein skimming has been made within the indicated salinity area from about 10 to about 50 parts per thousand, of which the bubble size would be between about 0.1 to about 1 mm. Fresh water also has a lower density than salt water, and this makes it more difficult to form small stable bubbles. The attached FIG. 12 shows an area for bubble sizes formed for different salinities of water. It has been less usual to try to remove protein from water having salinity less than about 5 to 10 parts per thousand.